Many interconnect structures found in the electronic industry comprise alternating layers of dielectric material and metal. One common interconnect structure comprises alternating layers of a polymeric material, such as polyimide, and copper. Such structures usually begin with a base substrate, such as a silicon wafer, which is coated with a layer of polyimide using a spin-on coating process, much like that used to apply a photoresist layer. Thereafter, the starting polyimide layer is soft baked, and then cured. The curing step imidizes (i.e., cross-links) the polymer chains of the polyimide material. The polyimide may be cured by exposure to heat, ultraviolet light, electron beam radiation, or a combination thereof.
A metal layer is then formed over the starting polyimide layer and patterned to provide the desired electrical interconnects for that layer. The metal layer may be formed by sputter coating a seed layer of material, such as chromium, followed by an electroplating process. The resulting metal layer may then be patterned by selectively etching the material through a patterned photoresist layer, as is well known in the semiconductor processing art. A second polyimide layer is then formed over the first metal layer, and then soft baked. However, before curing the second polyimide layer, the layer is usually patterned to form vias, or apertures, through the second polyimide layer to parts of the underlying first metal layer. These vias will enable vertical electrical connections to be made between signal lines in the first metal layer, and signal lines in a second metal layer which will be subsequently formed over the second polyimide layer. The vias may be made by the conventional photoresist/etching steps, or a photo-imageable polyimide may be used. The vias are then filled with metal, which may be done by an electroless plating or electroplating process. Thereafter, a second metal layer is formed and patterned, which may be done with the same steps used to form and pattern the first layer. The number of polyimide and metal layers constructed depends upon the design and purpose of the structure.
The multilayer ceramic art, which is a more mature technology, has a similar structure of alternating dielectric and metal layers. The dielectric layers are raw, un-fired sheets of ceramic material (so-called "green-sheets"), upon which the metal layers are formed by screening metallic paste over the raw sheets through patterns. After screening, the raw sheets are pressed together and heated to a high temperature to cure the raw sheets. The heating process is known as a "firing" step. In addition to signal lines, the multilayer ceramic structures use AC ground planes on either side of one or more of the metal signal layers, with a dielectric ceramic layer separating each AC grounding plane from each metal signal layer. These AC grounding planes are used to provide each signal line with a controlled impedance characteristic, and to reduce the coupling of electromagnetic energy between adjacent signal lines (so called "cross-talk") by establishing a desired electromagnetic field pattern between each signal line and the AC grounding layers. The controlled impedance characteristic also enables the designer to better estimate the signal propagation characteristics of the signal lines. The design of such structures is well known in the microstrip and slot-line arts, which are part of the electromagnetic wave propagation art.
Difficulty has been encountered in incorporating these AC grounding planes into metal/polyimide structures. The difficulty results from the fact that the polyimide layers readily absorb water at room temperature, and readily release the water as steam when the structure is heated, which may occur during operation of the substrate in the field from the power dissipation of integrated circuit components which are attached to the substrate. For polyimide layers which are disposed between two AC ground planes, the heated steam is trapped between the two metal layers and cannot readily escape. The steam accumulates in small pockets at the interfaces between metal and dielectric layers, where it builds up pressure. With sufficient pressure, the layers delaminate (e.g., separate at the interface) and weaken the structure. In the delamination process, the control impedance for the surrounding signal lines is disturbed, causing loss of a predictable behavior in the signal propagation characteristics, and some signal lines may even be ruptured, or broken. Moreover, cavities can develop between the metal and dielectric layers due to the delamination process. Steam can condense within these cavities, which act as a reservoirs, or "pools", of condensed water having dissolved ionic species therein. This condensed water can initiate conductor corrosion and device failure.
The above delamination process may also occur when an upper polyimide layer is being cured by a heat treatment, or by other heat treatments which may be needed in the manufacturing of the multilayer substrate. In addition to releasing steam, polyimide layers which may not have been fully cured may release gases that are generated as part of the imidization process. Additionally, various photo-active compounds in photo-imageable polyimides may undergo decompositions which create additional gases. Most other types of polymeric materials suffer the above same problems, so that switching to a different type of dielectric material does not solve the above problems.
The present invention seeks to alter the design of AC grounding planes, and similar structures such as power distribution planes, so as to prevent delamination of layers in such structures.